1. Field of the Invention
The present invention relates to a fabrication method for high voltage devices having at least one deep edge ring, particularly for high-voltage power devices implemented in MOS technology.
2. Discussion of the Related Art
For many high-voltage devices, particularly for those manufactured with MOS technology, where the output resistance is strongly influenced by the resistivity of the silicon the device is fabricated in, it is necessary to achieve high breakdown efficiency, that is to say, a high ratio between the measured breakdown voltage and the theoretical breakdown voltage, of a plane junction.
A first method currently in use to improve breakdown voltage entails the formation of "floating" rings separated from the body region but manufactured simultaneously with the body region of the high voltage device and having the same depth as the body. These rings therefore have a doping concentration that is the same as the doping concentration of the body region.
An improvement in breakdown voltage when using this method can be achieved by increasing the number of floating rings.
Increasing the number of floating rings increases the device area and presents difficulties, since the spacing between the various rings is typically fixed.
A second method being used to improve breakdown voltage entails the provision of structures having rings that are coupled to the body. In this case, since the doping concentration of the rings must be much lower than that of the body, the rings are produced in separate steps from the steps used to produce the body.
Due to this lower doping concentration of the rings with respect to the body, for the same thermal steps, the depth of the rings is less than that of the body, so that in the junction point between the body and the first ring a peak occurs in the electrical field that decreases the breakdown voltage at the edge of the device.
A third method being used to improve breakdown voltage and therefore the shape of the electrical fields entails providing one or more rings the junction depth whereof is greater than the depth of the body of the high voltage device. This allows a reduction in the electrical field in the critical point where the body and the ring join, thus increasing edge efficiency.
Both in the first and in the second methods described above, it is possible to use boron as a dopant without requiring high thermal budgets.
In the third method, instead, where the ring has a greater junction depth than the body, the ring is formed before the body and requires diffusion processes having a high thermal budget (i.e., long diffusion times and high temperatures such as, for example, several tens of hours at temperatures above 1100.degree. C.).
A drawback of the third method, in addition to the greater complexity of the manufacturing process, which is therefore more difficult to carry out in industrial practice, is that the doping profiles of the drain are altered dramatically due to diffusion of the substrate dopant into the epitaxial layer ("out diffusion").
One solution currently adopted to increase breakdown voltage consists in using aluminum as the dopant both for the body and for the rings. The implantation of aluminum ions leads to low electrical activity, which is a direct consequence of various phenomena, the most important of which is the escape of aluminum ions from the surface.
In order to overcome this drawback, layers of dielectric material are used as a covering to prevent out diffusion, particularly layers of oxide-nitride-oxide or SIPOS.
One object of the present invention is therefore to provide a fabrication method for high voltage devices with at least one deep edge ring having a high breakdown voltage but that only requires low ion implantation energies.
Another object of the present invention is to provide a fabrication method for high voltage devices with at least one deep edge ring that uses a single thermal process to form the body and the ring.
Another object of the present invention is to provide a fabrication method for high voltage devices with at least one deep edge ring that avoids the loss of aluminum from the surface of the silicon and therefore does not require high ion implantation energies.
Another object of the present invention is to provide a fabrication method for high voltage devices with at least one deep edge ring without having to resort to covering layers of dielectric.